The present invention relates to optical communications, and more particularly to wavelength division multiplexing (WDM) transmitters.
WDM techniques are finding increasing application in optical networks. When WDM is used, multiple optical signals at different wavelengths are combined on a single fiber. This type of operation greatly increases the data carrying capacity of a single fiber and also offers other implementation advantages.
A WDM transmitter accepts multiple digital data streams and uses them to modulate optical signals at disparate wavelengths. The transmitter includes a laser to generate a sinusoidal optical signal at each wavelength included in the WDM scheme. For each laser, there is a modulator that superimposes the digital data on the sinusoidal signal. The modulator may be integral with the laser or may control electrical input to the laser. A multiplexer is then typically used to combine the various signals onto a single fiber for communication through the WDM link.
Operation of the WDM link including both the transmitter and receiver(s) depends on a shared understanding of the arrangement of the multiplexed optical signals in the frequency domain. A WDM grid defines the number of signals, i.e., channels, the spacing between channels, and their exact position in the frequency domain. The frequency of each optical signal will depend on the transmission frequency of the corresponding laser. If this transmission frequency drifts from its desired value, communication is impaired. For example, the spectrum occupied by one WDM channel may overlap and degrade another WDM channel. Also, demultiplexers and other receiver components tuned to the WDM channel's desired wavelength rather than its actual wavelength will receive the signal at reduced power or not at all. For these reasons, WDM transmitter systems typically require measurement of laser transmission frequency in combination with feedback-based control.
To precisely control the transmission frequencies, each laser is typically equipped with a wavelocker device that samples the laser output beam and divides the sampled energy into two parts. A first photodiode measures the power, P1, of the first part, while the second part of the sampled energy passes through a filter having the desired transmission wavelength as its center frequency before being measured by another photodiode that gives its power, P2. The laser frequency then can be determined based on the ratio of P1/P2. A variable electrical current is then used to adjust the laser temperature to set the laser to its desired frequency. These wavelocker devices are expensive optical systems and must be provided for each WDM channel.
The trend in optical system development is toward greater and greater numbers of WDM channels at closer spacings. Accordingly, many current and future WDM systems are often referred to as DWDM (dense wave division multiplexing) systems. A greater density of channels in current and emerging DWDM systems tightens laser frequency tolerance requirements and necessitates ever more precise control. Increasing numbers of channels in DWDM systems directly correlates to increased numbers of wavelocker devices and thus greater expense and consumption of space. What are needed are cost efficient and space efficient systems and methods for laser frequency control in WDM systems.